1. Field of the Invention
The present invention generally relates to the technical field of mobile communications, and more particularly relates to apparatuses and methods for communication in variable bands.
2. Description of the Related Art
The Orthogonal Frequency Division Multiplexing (OFDM) scheme has various advantages such as high tolerance for multipath transmission channels and high efficiency of frequency utilization. In the OFDM scheme, data is mapped into a large number of subcarriers arranged orthogonally with each other, and inverse Fourier transform is performed on the mapped data pieces to derive a time-series signal and generate transmitted symbols for radio transmission. At the reception side, Fourier transform is performed on the symbols to derive the transmitted data pieces, and the individual data pieces associated with the respective subcarriers are restored to reconstruct the transmitted data.
FIG. 1 shows exemplary signals communicated in the OFDM scheme. Data to be transmitted is mapped into several subcarriers. In this case, one of the subcarriers corresponding to a central frequency f0 is not used for the data mapping and thus is a subcarrier without data. The central frequency corresponds to a carrier frequency, and thus even if data is mapped into it, the data cannot be transmitted with high quality due to a strong interference component caused there. The central frequency may be referred to as “DC subcarrier” or “DC offset”.
Meanwhile, in the third and subsequent generation mobile communication systems, it is assumed that user equipment (UE) or user apparatuses use a part or full range of the system band depending on conditions.
In an example illustrated in FIG. 2, the whole system bandwidth of 20 MHz is provided, and user apparatuses are allowed to use 10 MHz or 20 MHz for communication. A larger number of system bandwidths may be provided. For example, system bandwidths such as 1.25 MHz or 2.5 MHz may be provided. Utilization efficiency of radio resources may be improved through selection of suitable bands among various bands with greater and smaller bandwidths depending on communication environments and applications. For example, 3GPP TR25.814 V1.5.0 (May 26, 2006) discloses communication systems for communication in variable bands. The communications in variable bands used herein does not mean that user apparatuses are allowed to use the whole frequency in the bands. In fact, one or more of resource blocks included in a variable band assigned to a user apparatus are available for communication.
In the course of fundamental research on the present invention, the inventors had an idea that the communication in variable bands may be achieved in the OFDM scheme. If the idea can be realized, the above-mentioned outstanding advantages of the OFDM scheme, such as high tolerance over multipath interference and high efficiency of frequency utilization, can be applied to the communication in various bands, which may improve transmission efficiency. Also in this case, it must be taken into consideration the central frequency of a band used in communication should not be used for data transmission.
FIG. 3 shows exemplary communications in variable bands in connection with FIG. 2. In FIG. 3, the relationship between subcarriers and central frequencies is clarified. In the illustration, users A and D communicate with the bandwidths of 10 MHz and 20 MHz, respectively, and the respective central frequencies are the same frequency f0. Thus, data can be transmitted with subcarriers other than the central frequency f0 as is the case of conventional systems in accordance with the OFDM scheme. On the other hand, users B and C use 10 MHz similar to user A but have central frequencies different from user A. For convenience of explanation, the central frequency (DC subcarrier) of a band with 10 MHz for user B is denoted by “fB” while the central frequency of a band with 10 MHz for user C is denoted by “fC”. As stated above, no data can be mapped into the frequency fB in communications with user B. Similarly, no data can be mapped into the frequency fC in communications with user C. In communications with user D, however, the frequencies fB and fC do not correspond to the DC subcarrier and accordingly are available for data transmission. In other words, the frequency fB or fC can or cannot be used for the data mapping depending on the bandwidth for user apparatuses. Thus, frequency scheduling has to be performed by determining availability of individual subcarriers for the data mapping, which may not be desirable from the viewpoint of increasing the complexity of controlling base stations.